A bizarre neurological syndrome, in which patients are unable to control the actions of one of their own hands, is helping scientists to understand the basis of free will.

Patients with the phenomenon, known as anarchic hand, behave as if they have two streams of consciousness, or two separate wills, which compete with other.

This may lead to one hand "arguing" with the other, for example in choosing which television channel to watch or what to eat.

Neurologists believe there are regions in the human brain that regulate actions driven by inner will while inhibiting actions triggered by the environment.

How free are we?

When these areas are damaged, for example by a stroke or head injury, the person is left at the mercy of environmental triggers and their actions do not match their will.

It raises the question, "how free is our free will?" said Sergio Della Salla, Professor of Neuropsychology at the University of Aberdeen, UK. "It seems to demonstrate that self-ownership of actions can be separated from awareness of actions. Anarchic hand patients seem to be aware of the actions of their anarchic hand but they disown them."

Only about 40 cases of the condition have ever been documented.

Professor Della Salla told the British Association's Festival of Science about two patients currently under observation.

Violent actions

One patient, at dinner, to her dismay, saw her left hand taking some fish-bones from leftovers and putting them in her mouth.

Another patient complained that her hand did what it wanted to do, and tried to control its wayward behaviour by hitting the hand violently or shouting at it.

"The patients are aware of the bizarre and potentially hazardous behaviours of their hand but have great difficulty inhibiting it," said Professor Della Salla. "They often refer to the feeling that one of their hands behaves as if it has its own will but never deny that this capricious hand is part of their own body."

The researchers are now devising new experiments to test cognitive models of anarchic hand. They believe it could lead to a greater understanding of how the human mind works.